1. Field of the Invention
This invention relates to the use of isopalmitic acid esters of branched aliphatic polyols containing 2 to 6 primary hydroxyl groups as a base oil for lubricants for two-stroke engines.
2. Statement of the Related Art
In two-stroke engines, the lubricant is generally supplied in admixture with the fuel and enters the combustion chamber through slots via the crank casing. The lubricant lubricates the crankshaft and the cylinder and, at the same time, participates in the combustion process. For lubricants for two-stroke engines, it is important that there should be no deposits in the combustion chamber nor any incrustations around the outlet slot because coke-like deposits in the outlet slot reduce the performance of the engine to such an extent that it can no longer operate satisfactorily. Since the lubricant also takes part in the combustion process, the formation of residues on the spark plugs must be avoided. Accordingly, base oils for two-stroke engine lubricants are expected to undergo complete combustion. In addition, the viscosities of the base oils should lie in certain ranges so that adequate lubrication is always guaranteed both at high temperatures and at low temperatures and even at high speeds. Users of two-stroke engine lubricants require base oils having viscosities in the range from 8 to 15 mm.sup.2. s.sup.-1 at 100.degree. C. in accordance with DIN 51 562, Part 1, and below 11,000 cP at -25.degree. C. in accordance with ASTM D 2983. In addition, every base oil must be readily miscible with or soluble in gasoline over wide temperature ranges. Thus, no crystallization of the base oil should occur, even over periods of several days at low temperatures.
In principle, isopalmitic acid esters of branched alcohols are known as lubricants from DE-A 23 02 918. In view of their favorable low-temperature and viscosity behavior, they are recommended therein as sole constituents or, in admixture with mineral oils and ester oils, as hydraulic oils and, generally, as lubricants. Further particulars of applications are not provided although the field of lubricants is extensive and the requirement profile of lubricants differs considerably according to the particular application.
DE-A-37 12 133 describes lubricants based on mineral oil and/or synthetic oils containing polyol esters, such as pentaerythritol tetraisopalmitic acid ester. By virtue of the thermally stable polyesters, these lubricants are suitable for the permanent lubrication of heavily stressed engines, turbines, antifriction bearings and constant-velocity joints. The suitability of the lubricants for use in diesel engines and aircraft turbines is particularly emphasized. There is no reference to two-stroke engines as a potential application.
Commercially available base oils for two-stroke engine lubricants include inter alia the trimethylol propane esters of branched carboxylic acids marketed by Unichema under the name Priolube.RTM. 3999. Although Priolube.RTM. 3999 largely satisfies the requirement profile of two-stroke base oils, there is still a need for base oils for two-stroke engine lubricants which have lower viscosities at -25.degree. C. (in accordance with ASTMD 2983) to guarantee improved lubrication during the cold-starting of two-stroke engines.
The problem addressed by the present invention was to provide base oils for two-stroke engine lubricants which would be miscible with gasoline and which would not have any tendency to crystallize, even at low temperatures. In addition, the base oils would not have any tendency to form unwanted residues or coke-like deposits during lubrication and combustion. Finally, they would have low viscosities, even at low temperatures (below 10,000 cP at -25.degree. C. in accordance with ASTM D 2983).